Certain metal carbides display exceptional hardness and thermochemical stability. In particular the carbides of hafnium, niobium, tantalum, titanium and zirconium are among the most refractory materials known and are generally stable in severe chemical environments. Coatings of metal carbides have thus found application in the aerospace, nuclear and metallurgical industries. For example, high temperature nuclear reactors use nuclear fuel particles of uranium or oxides of nuclear fuel material which are encased in a protective coating usually of pyrolytic carbon with an outer metal carbide layer. The protective coating has the function of retaining the fuel and fission products during nuclear burn up within the individual particles themselves. The coating must endure the fuel element production and undergo no damage during nuclear burn up. Failure of the coating results in the release of the fission products.
It is known to decompose a hydrocarbon gas such as methane to form a coating of pyrolytic carbon on a substrate in a fluidized bed furnace. It is also known to use standard chemical vapor deposition in combination with a fluidized bed reactor to form a metal coating on particles suspended in the fluidized bed. In this regard fluidized bed systems have been widely used in many industrial applications to form a coating of metal on a substrate. The metal coating formed by the decomposition of a gas forms a mechanical bond at the surface of the substrate which is acceptable for most applications but not in situations where the coating is subjected to extreme environmental conditions particularly in a nuclear reactor. Accordingly, there is an existing need for a method for forming a metal carbide coating which is chemically bonded to the substrate surface and which can withstand the hostile conditions in a nuclear reactor.
A high temperature coating process and apparatus has been developed in accordance with the present invention which combines fluid bed reactor technology with chemical vapor deposition techniques to produce a chemically bonded coating of a metal carbide on a substrate containing carbon or graphite. The carbon or graphite containing substrate may be in the form of particles which are fluidized to form the fluidized bed or may be added to the fluidized bed. The process of the present invention may be used to produce a single or multilayer coating and is particularly suited for forming a dual coating with one layer of pyrolytic carbon and another of metal carbide. Moreover, the metal carbide coating has a uniform thickness with a density substantially close to theoretical density. In accordance with teaching of the present invention particles of nuclear fuel may be encapsulated within a metal carbide diffusion barrier capable of withstanding the condition in a nuclear reactor at temperatures in excess of 2300.degree. C.